Advances in LED (light-emitting diode) technology have led to the development of LEDs with a satisfactorily high light output, making them an interesting alternative to incandescent or fluorescent lamps. An LED lighting device can easily be designed to exceed an efficacy of 100 μm/W. Furthermore, LEDs are more efficient and more reliable than conventional lamps, and have a longer lifetime. Therefore, using LEDs to replace conventional lamps contributes to a reduction in energy consumption and reduced emissions from power plants. Packages or chips containing series-connected LED junctions (to form an LED string with a high forward voltage) are suitable for low-cost general lighting applications, and LED devices directly running off an AC mains supply—called ACLEDs or mains-compatible LEDs (MCLEDs)—are currently available. However, there are some drawbacks related to direct mains driving. Firstly, the current waveform given to the ACLED has a high peak value compared to the average value. Hence, the ACLEDs are driven with a reduced efficiency due to ‘droop’. Secondly, current flow through the ACLED package is only possible when the instantaneous mains voltage is higher than the forward voltage of the package's LED string. Therefore, during relatively ‘long’ periods, no light is emitted. This is perceived as an annoying ‘flicker’, so that such lamps are unacceptable for applications such as indoor lighting.
The flicker problem can be solved by using a rectifier and a capacitor, however, a high voltage LED string will still be required, since the working voltage of the LED must match the resulting rectified mains voltage. Usually, an LED chip comprises one or more LED junctions connected in series such that the sum of the forward voltages matches the supply voltage. Evidently, the more junctions of a given size are connected in series to act as an LED load, the higher will be the power consumption of the LED load. As the skilled person knows, to obtain an LED load with a high forward voltage and low power consumption, a small junction size would be required. However, very small junctions are expensive to manufacture, and, because of the smaller active area, the overall efficiency obtainable by such a device is unsatisfactory. Therefore, a low power LED package for operating off a 230 V mains supply cannot be realised and operated economically. To operate a more economical, commercially available device, for example a device rated for a 110 V mains, additional circuitry such as a transformer or a capacitive power supply must be used to convert the high input voltage to the necessary low output voltage. Such circuits are lossy, expensive, bulky and heavy and are associated with an unacceptably low power factor of less than 0.5.
However, 0.5 is the minimum power factor specified by some energy labels for LED lamps. But even without this requirement, it is desirable to achieve a high power factor while minimizing losses and the cost required to achieve such a high power factor. One reason for desiring a higher power factor is that, at a fixed real power consumption and a low power factor, the input current of a load (and therefore also the current stress for some components) is typically high. To achieve at least the desired minimum power factor, the known circuits require very precise matching of the components. However, this is made very difficult owing to the unavoidable component tolerances and the variations in voltages (voltage bins) even for identical LED packages. In an alternative approach, JP 5709736 describes the use of a capacitive split circuit with two or more switches for obtaining a step-down voltage. However, in the circuit described, at least one of the switches will be subject to extreme voltage stress during mains voltage transients, so that the switch may ultimately fail unless it is provided with additional overvoltage protection. Also, at least two switches are necessary, and these must be precisely synchronized to the input signal.
Therefore, it is an object of the invention to provide an improved adaptive circuit that avoids the problems mentioned above.